The teachings herein relate generally to techniques for safe and controllable de-energizing of an electrical generator. In this context, de-energizing of an electrical generator pertains to countering the motive forces on the generator shaft, the reduction of rotational energy stored in the rotational mass of the generator, and the reduction of electromagnetic field energy stored in the generator.
In embodiments where control over the motive force is limited or not possible, methods for de-energization of the generator in a controlled manner must be provided. One example includes generators driven by a wind turbine. For example, in instances where high winds prevail, stopping the wind turbine could exert unreasonable or excessive forces upon the components of the wind turbine.
Some attempts to provide for effective de-energization have employed mechanical brakes. However, mechanical brakes can be large, expensive and wear quickly if the braking event occurs often. Such systems require periodic maintenance and testing to ensure reliability, and this causes at least some loss in production. Typically, the mechanical brake has a relatively long response time (tens of seconds). Under some severe conditions, the long response time can result in over-speed of the electric generator. Controlling the de-energization process using mechanical brakes is difficult. Further, such systems must be fail-safe and meet high reliability standards.
In installations with limited or no control over the motive force, the speed of the electric generator has typically been reduced by a combination of a crowbar circuit and a mechanical brake or a dynamic brake in a dc-link of a power converter.
What is needed is a redundant, controllable and cost-effective braking system for an electric generator. Preferably, the braking system provides for de-energizing of the electrical generator in a rapid, safe and reliable fashion.